Method of decontaminating radioactive wastes

ABSTRACT

LIQUID RADIOACTIVE WASTE MATERIALS, INCLUDING THOSE CONTAINING RADIOACTIVE RUTHENIUM IN CHEMICAL COMBINATION WITH OTHER ELEMENTS, ARE DECONTAMINATED BY ADJUSTING THE PH THEREOF TO A VALUE BETWEEN 1 AND 3, PERCOLATING THEM OVER FINELY DIVIDED ACTIVE IRON AT A TEMPERATUE BELOW 70*C., RAISING THE PH TO A VALUE BETWEEN 8 AND 10, AND THEN BY CONTACTING THE LIQUID WITH SYNTHETIC ZEOLITES.

United States Patent US. Cl. 252301.1 WD 6 Claims ABSTRACT OF THE DISCLOSURE Liquid radioactive waste materials, including those containing radioactive ruthenium in chemical combination with'other elements, are decontaminated by adjusting the pH thereof to a value between 1 and 3, percolating them over finely divided active iron at a temperature below 70 C., raising the pH to a value between 8 and 10, and then by contacting the liquid with synthetic zeolites.

Processes for decontaminating liquid radioactive waste materials are intended to separate the radioactive ingredients from the liquid so as to make the liquid residue harmless. The radioactive portions are then concentrated in solid form into a small volume which is then encased, for example in concrete, to be held in storage until the radioactivity thereof has declined to harmless levels.

The methods currently employed for decontamination of radioactive wastes reside primarily in processes for removing the radioactive ingredients by means of a precipitate formed in situ within the liquid. These methods are effective for most of the radioactive materials which are present, with the exception however of ruthenium. Ruthenium escapes almost wholly these precipitation processes because it is present in the liquid waste products in complex chemical forms and in particular in the form of a radical RuNO which is tied to various ligands. The elimination of ruthenium has heretofore required a preliminary treatment to break down or transform these compounds so as to give them a chemical activity sutficient to make them form compounds which can be eliminated by precipitation or by fixation on zeolites.

The present invention pertains to a method of treating liquid radioactive wastes which achieves an improved degree of decontamination, especially for such wastes containing ruthenium and wastes which even without ruthenium have a low initial level of radioactivity.

In accordance with the invention the pH of the liquid Waste is adjusted to a value between one and three and the liquid is percolated, at a temperature not above 70 C., over active, oxide-free, iron having a high surface area per unit volume or per unit mass. The pH is then raised to a value between 8 and 10 and the effluent is subjected to a treatment with zeolites.

The process of the invention combines treatment by finely divided active iron and treatment with zeolites. It has been found that for a given initial level of radioactivity due to ruthenium, the decontamination factor (reduction in radioactivity) achieved by treatment with zeolites is substantially improved when the treatment by means of zeolites is preceded by a treatment with finely divided active iron. The combination of the two treatments is a synergistic one.

As applied to a liquid radioactive waste which has not been treated, the method of the invention thus achieves a large overall decontamination factor, greater than 1,000; and in particular a reduction in radioactive ruthenium content by a factor in excess of 200. That is, the total radioactivity of the waste is reduced by the method of 3,658,714 Patented Apr. 25, 1972 the invention to less than one one-thousandth of its initial value, and the ruthenium content thereof is reduced to less than one twohundredth of its initial value. Contact time between raw Waste and activated iron must be long enough to allow the pH of the percolated solution to rise 5 at least.

It is also noteworthy that, contrary to known processes, the process of the invention preserves this high decontamination factor even when applied to wastes which have been chemically treated to reduuce the radioactive level thereof to low levels, for example to values of 2 l0 curies per cubic meter or less. It is always the last traces of radioactivity which are the most difiicult to eliminate.

The absolute level of radioactivity achieved by the process of the invention is thus better, i.e. lower than in known processes.

In general the process of the invention makes it possible to reduce the concentration of radioactive elements obtained from the fission of uranium 235 down to the maximum values permitted by the International Commission for Protection Against Radiation.

The process of the invention is particularly advantageous when ruthenium is present among the radioactive elements to be eliminated.

The finely divided iron used is preferably in the form of a very fine wool w-hose fibers preferably have a diameter of 50 microns or less and over which the liquid radioactive waste to be decontaminated is caused to flow. Consistently with the invention however powdered iron may be used instead, for example having an average particle size of microns or less, obtained by reduction by means of hydrogen and dispersed through the effluent.

In order for the process to be successfully practiced it is necessary for the iron to be active, free of surface oxide. For this purpose applicants prefer to degrease the finely divided iron by means of an organic solvent such as acetone, alcohol, trichlorethylene or the like, and to wash it with lightly acidulated distilled water, acidulated for example by means of nitric acid. The iron is then allowed to rest for at least 12 hours in a solution containing at least one salt of the metals taken from the group consisting of copper, nickel, cobalt and manganese.

Another mode for rendering the iron active which can be used in addition to or in place of that just described comprises heating the iron to 900 C. in a stream of hydrogen.

The zeolites used in the second step of the method of the invention are desirably artificial zeolites having the form of a powder with a large surface area per unit volume or weight, in excess of 50 square meters per gram and indeed preferably in excess of square meters per gram. The zeolites are chosen according to known methods as a function of the radioactive elements which accompany the ruthenium and which are to be removed.

According to a preferred mode of practice of the invention, the operation is a continuous one with continuous feed of the liquid to be decontaminated and with continuous withdrawal of the decontaminated effluent through a filtering device.

When the efiluent to be treated contains a substantial amount of zirconium, for example 50 milligrams per liter or more, it is advantageous to precipitate the zirconium as a phosphate or hydroxide before carrying out the first step of the invention.

The invention will now be further described in terms of a number of non-limitative examples.

EXAMPLE I The radioactive liquid waste to be treated was an effluent from a nuclear research laboratory. Its specific activity, amounting to 2x10" curies per cubic meter, was attributable to nine different radioactive nuclides as follows:

Curies per cubic meter A quantity of this waste was decontaminated at room temperature by the process of the invention. That is, with its pH adjusted to a value between 1 and 3, it was passed through a column containing 2 grams of iron wool of 50 micron fiber diameter, rendered active by means of a copper salt as above described. The pH of the percolated solution was 5.5. The pH of the liquid was then raised to 8, and the liquid was then subjected to the action of, Le. contacted with, a synthetic zeolite. The radioactivity of the liquid after this treatment amounted to l.4 lcuries per cubic meter, of which 1.3 1() was attributable to its ruthenium content. The total decontamination factor achieved by the process of the invention was consequently 3.2 /1.4 10 or about 2,300, and the decontamination factor for the ruthenium content was 0.5 l0 /1.3 X 10- or about 380.

In contrast, another quantity of the same radioactive liquid waste, when subjected to the usual series of chemical treatments of the prior art for coprecipitation of the radioactive ingredients along with ferrous hydroxide, nickel ferrocyanide and calcium carbonate, was reduced in radioactivity only to 2X10 curies per cubic meter.

Again in contrast, another quantity of the same radioactive efiiuent was passed through a column containing two grams of iron wool of 50 micron fiber diameter activated by means of copper. The pH was then adjusted to 8 and coprecipitation of the radioactive contaminants was carried out with ferrous hydroxide, then with nickel ferrocyanide and calcium carbonate. The final activity level of radioactivity reached was 1.2 10- curies per cubic meter, again very substantially higher than the level of 1.4 10- curies per cubic meter reached in Example I of the invention.

EXAMPLE H In this example the starting material was a radioactive liquid waste which had already been subjected to the decontamination processes of the prior art, namely coprecipitation and ion exchange with organic resins and inorganic ion exchangers. By means of these prior art processes the radioactivity of the liquid had been reduced to 5X10- curies per cubic meter and could not be further decontaminated by prior art processes, except that part of the liquid might be evaporated olf to provide an evaporated fraction of reduced radioactivity and a residue of increased radioactivity. By reason of its low radioactivity, this liquid waste constituted a very unfavorable starting material. A quantity thereof subjected directly to treatment by zeolites showed no substantial further reduction in radioactivity. Another quantity thereof, when subjected to the process of the invention, was reduced in radioactivity by a further factor of fifteen, to a level of 3 X10 curies per cubic meter.

EXAMPLE III A radioactive liquid waste having an initial radioactivity due to ruthenium of 5x10 curies per cubic meter was subjected to the process of the invention. In the first step thereof, the efiluent was passed over active iron in accordance with the invention, and its activity was thereby reduced to 10- curies per cubic meter. The efiluent was then subjected to treatment by zeolites in accordance with the invention, and its ruthenium radioactivity was thereby reduced by a further factor of 20 to 5X10 curies per cubic meter.

In contrast, a comparable efiluent when treated according to the prior art by coprecipitation was reduced in radioactivity to 10- curies per cubic meter. The efliuent so treated was then further treated with the same zeolites as those used in Example III. This treatment with zeolites reduced the radioactivity however by no more than a factor of 10, from 10- to 10- curies per cubic meter. Treatment of radioactive wastes by zeolites is thus seen to be much more effective when it is carried out in accordance with the invention, after an initial treatment with active iron.

We claim:

1. The process of decontaminating liquid radioactive waste material comprising contacting the material with finely divided activated iron and thereafter contacting the material with at least one zeolite.

2. A process according to claim 1 in which the pH of the liquid is adjusted to a value between 1 and 3 prior to the first contacting step and in which the pH of the liquid is adjusted to a value between 8 and 10 after the first contacting step and prior to the second contacting step.

3. A process according to claim 1 in which the iron has been degreased and immersed in a solution containing a salt of a metal from the class consisting of copper, nickel, cobalt and manganese.

4. A process according to claim 1 including the preliminary step of heating the iron to a temperature of about 900 C. in a substantially hydrogen atmosphere.

5. A process according to claim 1 in which the zeolite is a synthetic zeolite.

6. A process according to claim 1 in which the radioactive waste material is maintained in contact with the finely divided iron, long enough to allow the pH of the material to rise to at least 5.0.

References Cited UNITED STATES PATENTS 6/1959 Lowe 23337 X 9/1957 Alberti 210-24 R. L. TATE, Assistant Examiner U.S. Cl. X.R. 

